1. Field of the Invention
The invention relates to a cobalt rare-earth alloy for permanent magnets, and more particularly to a binder system for cobalt rare-earth permanent magnets.
2. DESCRIPTION OF THE PRIOR ART
A new class of materials exhibiting permanent-magnet properties has been introduced in the last few years. These materials are basically alloys containing approximately 10 to 25 atomic percent of rare-earth elements and 75-90 atomic percent cobalt, a part of which may be replaced by the elements manganese, iron and/or nickel. An additional alloy partner in these alloys can be copper. The materials may contain a single rare-earth element or a mixture of several rare-earth elements may be used.
From these alloys, which have high magnetocrystalline anisotrophy, permanent magnets can be produced in the following manner: The alloys are comminuted into powder form, binder material is added if desired, the powder mixture is exposed to a magnetic field, and the blend is then formed into a rigid magnet by pressing, sintering, or hardening of the binder. It is furthermore possible to achieve permanent-magnet behavior in the massive state by adding copper to the alloy and subjecting the latter to an appropriate heat treatment. In this case, the magnets with the best properties should in principle be castings of the alloy. In the case of powder-metallurgical magnet preparation from copper-free rare earth alloys, the best magnets are made by sintering or hotpressing to the highest possible density. In both cases the magnets have the disadvantage of poor mechanical properties. They are brittle and subject to internal cracking, and they either cannot be machined at all, or only by grinding or abrading.
The ferrite and Alnico materials, which are the permanent magnets predominantly used in technological applications at the present time, have the same disadvantages. An improvement of these mechanical properties is therefore desirable.
An Alnico system is seen, for example, in U.S. Pat. No. 2,724,174 to Mendelsohn, which discloses that moldable compositions can be prepared by coating finely-divided Alnico with a low melting point metal or alloy and producing intermetallic bonding. The product, which is obtained after a heating step in which the low temperature metal is melted, is still a powder which is suitable for subsequent forming into a magnet by the usual and previously known steps of magnetic aligning and compacting under pressure without adding any additional solder or binding material. The process applies to the magnetic materials family known as Alnico, the iron content of which appears to play an essential role in the successful formation of the soft-metal surface coating by the process described in the patent (see column 1, lines 65 through 69, and column 2, line 40).
Alnico is noted to be a solid solution alloy unlike the intermetallic compounds of the instant invention which are based on cobalt and substantial quantities of rare-earth metals. The Alnico alloys are based on iron and contain several other important constituents such as nickel and aluminum which are not normally present in cobalt rare-earth magnets. The rare-earth component of the cobalt rare-earth magnets is, of course, not found in Alnico magnets.
The composition differences between the Alnico and rare-earth cobalt magnets unfortunately render the Alnico technology inapplicable to the rare-earth cobalt magnets, at least on a direct and predictable basis. Along these lines, it is noted that the mechanism of bond formation in the Mendelsohn Alnico magent involves the formation of intermetallic compounds between the bonding metal and one or more of the constituents of the Alnico, probably the iron, a material not found in cobalt rare-earth magnets.
It is thus found that the availble technology has not served to overcome the problems of the commercially available magents made from rare-earth cobalt alloys by sintering. The magnets are inherently brittle intermetallic compounds and inevitably have high internal stresses and often microcracks from the quenching or rapid cooling, after sintering, which is necessary to develop high coercive force. They are especially sensitive to impact when dropped or allowed to snap against each other or onto iron parts under the strong forces of magnetic attraction. This is often a considerable disadvantage in the assembly of magnetic devices, as well as during machining and handling in general.
The sintered magnets are very hard and can only be machined by the methods used for hard ceramics, such as grinding or abrasive cutting, or by the slow and costly electric spark erosion technique. Sintered magnets, especially those of larger size, also often have nonuniform magnetic properties throughout their volume. This is especially true for the intrinsic coercive force and the loop squareness, both of which depend strongly on the cooling rate which can vary significantly within a magnet.
Conventional sintered ceramic magnets (barium or strontium ferrite) share many of the same disadvantages with the rare earth magnets, although their brittleness and susceptibility to breaking are not as extreme. Methods were devised of regrinding the ferrite material to a coarse powder and bonding the particles into a body of the desired shape, using an organic binder such as rubber, a phenolic resin, and the like, in order to solve the foregoing problems. The ferrites, being oxides, are quite insensitive to the exposure to elevated temperatures, oxygen and corrosive chemicals during processing, and to exposure to air and moisture during later use.
The ceramic magnet bonding methods may be applied to rare-earth-cobalt powders and produce magnets that are neither brittle nor sensitive to impact, that are more or less flexible, and that can easily be cut with normal steel tools. However, the disadvantages of such organic-bonded magnets are:
a. they are quite soft -- their mechanical strength properties are determined by the binder and are insufficent for many purposes -- small compressive or tensile stresses cause large elastic deformation or plastic flow;
b. the highest use temperature is determined by the chemical and structural stability of the binder-typical temperature limits are in the range of 60.degree. to 130.degree. C; and
c. organic binders cannot prevent the slow but progressive loss of coercive force which is known to occur when unprotected rare-earth cobalt powders are aged in air.